The comparative opacity to visible light of even thin layers of whole blood is detrimental to the measurement of oxygenation of intact red cells by conventional spectroscopic methods. In a medical emergency the time required to make such a determination can become important. The visible spectrum of opaque samples can be determined by photoacoustic spectroscopy (PAS). Furthermore, the time required to obtain such a visible spectrum can be drastically reduced to the order of a few seconds by the use of a Michelson interferometer and fast Fourier transform (FFT) techniques. Such an instrument will be assembled and used to investigate the dynamic behavior of intact red cells exposed to oxygen. Transient phenomena with half lives as short as microseconds will, in principle, be measurable.